A typical Doherty amplifier has a main (carrier) amplifier biased to operate in a linear mode such as Class AB mode and a peaking (or auxiliary) amplifier biased to operate in a non-linear mode such as Class C mode. The signal input to the Doherty amplifier is split to each amplifier, and the amplified signals are recombined using a Doherty combiner. Both amplifiers are operational when the input signal peaks, and are each presented with the optimum load impedance to yield maximum power output. As the input signal decreases in power, the peaking amplifier turns off and only the main amplifier operates. At these lower power levels, the Doherty combiner presents the main amplifier with a modulated load impedance that enables higher efficiency and gain. This results in an efficient solution for amplifying complex modulation schemes employed in current and emerging wireless systems e.g. such as WCDMA (Wideband CDMA), CDMA2000, and systems employing Orthogonal Frequency Division Multiplexing (OFDM), such as WiMAX (Worldwide Interoperability for Microwave Access) and the Long-Term Evolution (LTE) enhancement to the UMTS (Universal Mobile Telecommunications System) standard.
However, if high efficiency at a high OBO (output back-off) is required as is the case with many high peak-to-average power (PAR) applications, a highly asymmetric ratio between the size of the main and peaking amplifiers is required. With such an architecture, the efficiency between the peak OBO point where the main amplifier is conducting and the peaking amplifier is not conducting, and the peak power point where both amplifiers are conducting degrades significantly which is undesirable. A three-way Doherty architecture can be used to overcome this problem.
A three-way Doherty amplifier circuit typically includes a main amplifier which operates in a linear mode (e.g. Class AB mode) and two peaking amplifiers which operate in a non-linear mode (e.g. Class B or Class C mode). The three-way Doherty circuit has three power operating points: a peak power point where all three amplifiers are conducting; a first peak OBO point (back-off 1) where the main amplifier and the first peaking amplifier are conducting and the second peaking amplifier is not conducting; and a second peak OBO point (back-off 2) where the main amplifier is conducting and both peaking amplifiers are not conducting. Each amplifier stage is typically optimized as a 50Ω block, and the Doherty combiner is designed to provide the correct load impedances to each amplifier at back-off 1, back-off 2 and full power.
Each amplifier is conventionally connected to the Doherty combiner using an impedance match device such as an impedance transformer. The output match devices which connect the peaking amplifiers to the Doherty combiner cause an off-state impedance spreading effect across frequency when the peaking amplifiers are not amplifying. The off-state impedance spreading changes the VSWR (voltage standing wave ratio) seen by the main amplifier across frequency, and that de-tunes the main amplifier from the optimal load over a wide bandwidth. This in turn limits the overall bandwidth of operation for the three-way Doherty amplifier circuit. Doherty amplifier circuits are typically designed for a specific narrow frequency range of operation such as 1805-1880 MHz, 1930-1990 MHz, etc. Narrow band circuits are affected by the off-state impedance spreading and therefore cannot be operated across wider bandwidths.